Change Canvas.Left property in code behind?

Question

I have a rectangle in my XAML and want to change its Canvas.Left property in code behind:

<UserControl x:Class="Second90.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300" KeyDown="txt_KeyDown">
    <Canvas>
        <Rectangle 
            Name="theObject" 
            Canvas.Top="20" 
            Canvas.Left="20" 
            Width="10" 
            Height="10" 
            Fill="Gray"/>
    </Canvas>
</UserControl>

But this doesn't work:

private void txt_KeyDown(object sender, KeyEventArgs e)
{
    theObject.Canvas.Left = 50;
}

Does anyone know what the syntax is to do this?

score 170 · Accepted Answer · edited Apr 17 '12 at 17:58

170

Canvas.SetLeft(theObject, 50)

edited Apr 17 '12 at 17:58

answered Feb 12 '09 at 14:24

AnthonyWJones

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+1, gotta love type safety. I am curious though why SetLeft takes a UIElement instead of a DependencyObject – JaredPar Feb 12 '09 at 15:26
4

@JaredPar: at a guess I would say that since SetLeft is specifically a method of Canvas it understands what types it would make sense give a Left property to. It deems this to be UIElement, this perhaps increases the detection of faulty code where accidentally the wrong variable is passed to it. – AnthonyWJones Feb 12 '09 at 16:00
https://msdn.microsoft.com/en-us/library/windows.ui.xaml.controls.canvas.left.aspx Canvas.Left is an attached property, which supports a XAML usage. When setting this property in code, use SetLeft instead. – Yury Schkatula Mar 04 '15 at 22:45

JaredPar · Answer 2 · 2009-02-12T15:15:38.030

57

Try this

theObject.SetValue(Canvas.LeftProperty, 50d);

There is a group of methods on DependencyObject (base of most WPF classes) which allow the common access to all dependency properties. They are

SetValue
GetValue
ClearValue

Edit Updated the set to use a double literal since the target type is a double.

edited Feb 12 '09 at 15:15

answered Feb 12 '09 at 14:22

JaredPar

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Thanks, to get this to work I had to cast the integer: theObject.SetValue(Canvas.LeftProperty, (double)50); – Edward Tanguay Feb 12 '09 at 15:12
No, for this specify numeric constant in double format: heObject.SetValue(Canvas.LeftProperty, 50.0); – Budda Aug 16 '10 at 23:24

score 12 · Answer 3 · answered Aug 16 '10 at 23:23

12

As we are changing the property of the 'object', it would be better to use method suggedte by JaredPar:

theObject.SetValue(Canvas.LeftProperty, 50d);

answered Aug 16 '10 at 23:23

Budda

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Change Canvas.Left property in code behind?

3 Answers3

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